Loud exhaust rattle/grinding

Where is the noise coming from?!?!?!?!?!!?!!?!?

I suggest you have your car checked as soon as possible - while a rattling exhaust isn't desperately urgent it could be the cat breaking up which might later restrict the flow of gases. Most importantly excessively loose timing belts or chains tend to make most noise when the engine is idling. If the noise is being caused by this delay could be very expensive.

no 4-3 or 3-2 downshift
uncontrolled downshifts outside range of kickdown switch
poor acceleration from stopped position
parking pawl will not engage
selector lever does not engage in r or p
engine will not start in p or n position
smoke in exhaust (fluid loss)
fluid loss between torque converter & primary pump
howling noise when changing gears (under full load)
howling noise which increases as engine rpm increases
1st gear & reverse too loud
3rd gear too loud
rattling noise at 1500 rpm in all positions except r
light grinding noise in p & n positions
rolling noises when driving in reverse
vacuum control circuit
Vacuum Control Valve (E300D & S350D)
Vacuum Amplifier (E300D & S350D)

The rattle is caused because the rpm is lower. When stopped in drive with one foot on the brake, press the throttle down slowly until the vibration stops and the noise goes away. Cause? Weak or bad motor mounts, loose bolts or parts.

I take it that this is not stock exhaust, you mentio ed flex. If this is the case you will need to completely isolate the exhaust from all other components using rubber hangers. There can be NO contact with metal on metal, or you will get vibrations. Let me know if further assistance is needed.

hi this sounds like to me you might have a loose baffel in the box, the reason why the sound dissapates when you get to a certain speed is because the baffel is pushed up or down due to the pressure when you slow down the pressure then releases the baffel and it starts rattling again, you might also have a broken clamp on your exhaust or even a missing rubber bush check these out first...good luck

see if te noise is there when stationary. if so hold the revs around 1500 to 2000 rpm if it rattles around that, then it is the heat sheild on the down pipe section thats broken free.

several things could be wrong are you using 91 octane fuel or less? also it might be a timing issue you might be too far advanced in timing ... this causes preignition.. causing a rattle also cheaper fuel will do the same effect.. thanks Jerry holler if you need more.. also was the noise loud or just a mild noise ??

If this sounds like a tin can rattle it is likely a loose heat shield on top of the catalytic converter. They come loose after time and corrosion take their toll.

the heat shield on the exhaust manifold